Driving methods for color display device

ABSTRACT

The present invention is directed to driving methods for a color display device which can display high quality color states. The display device utilizes an electrophoretic fluid which comprises three types of pigment particles having different optical characteristics.

This application claims the benefit of U.S. Provisional Application Nos.61/887,821, filed Oct. 7, 2013; 61/925,055, filed Jan. 8, 2014;61/942,407, filed Feb. 20, 2014; 61/979,464, filed Apr. 14, 2014; and62/004,713, filed May 29, 2014. The contents of the above-identifiedapplications are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention is directed to driving methods for color displaydevices to display high quality color states.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. Themost common approach is to add color filters on top of black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. When a red color is desired, the green and blue sub-pixels areturned to the black state so that the only color displayed is red. Whena blue color is desired, the green and red sub-pixels are turned to theblack state so that the only color displayed is blue. When a green coloris desired, the red and blue sub-pixels are turned to the black state sothat the only color displayed is green. When a black state is desired,all three-sub-pixels are turned to the black state. When a white stateis desired, the three sub-pixels are turned to red, green and blue,respectively, and as a result, a white state is seen by the viewer.

The biggest disadvantage of such a technique is that since each of thesub-pixels has a reflectance of about one third (⅓) of the desired whitestate, the white state is fairly dim. To compensate this, a fourthsub-pixel may be added which can display only the black and whitestates, so that the white level is doubled at the expense of the red,green or blue color level (where each sub-pixel is now only one fourthof the area of the pixel). Brighter colors can be achieved by addinglight from the white pixel, but this is achieved at the expense of colorgamut to cause the colors to be very light and unsaturated. A similarresult can be achieved by reducing the color saturation of the threesub-pixels. Even with these approaches, the white level is normallysubstantially less than half of that of a black and white display,rendering it an unacceptable choice for display devices, such ase-readers or displays that need well readable black-white brightness andcontrast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an electrophoretic display fluid applicable to thepresent invention.

FIG. 2 is a diagram depicting an example of driving scheme.

FIG. 3 illustrates a typical waveform for driving a pixel from a whitestate to a red state in a color display device.

FIG. 4 illustrates a first driving method of the present invention.

FIGS. 5 and 6 depict driving sequences utilizing the first drivingmethod of the present invention.

FIG. 7 illustrates a second driving method of the present invention.

FIGS. 8 and 9 depict driving sequences utilizing the second drivingmethod of the present invention.

FIGS. 10 a and 10 b illustrate a third driving method of the presentinvention. FIG. 10 a demonstrates the relationship of applied drivingvoltage vs. optical state performance (a*), based on the waveform ofFIG. 3, and FIG. 10 b demonstrates the relationship of applied drivingvoltage vs. optical state performance (a*), based on the waveform ofFIG. 4.

FIG. 11 illustrates a fourth driving method of the present invention.

FIGS. 12 and 13 depict driving sequences utilizing the fourth drivingmethod of the present invention.

FIG. 14 depicts a typical waveform for driving a pixel to a black statein a color display device.

FIG. 15 illustrates a fifth driving method of the present invention.

FIG. 16 depicts a driving sequence utilizing the fifth driving method ofthe present invention.

FIG. 17 depicts a typical waveform for driving a pixel to a white statein a color display device.

FIGS. 18 a and 18 b illustrate a sixth driving method of the presentinvention.

FIGS. 19 a and 19 b depict driving sequences utilizing the sixth drivingmethod of the present invention.

FIG. 20 a diagram depicting another example of driving scheme.

FIG. 21 illustrates a typical waveform for driving a pixel to anintermediate color state in a color display device.

FIG. 22 illustrates a seventh driving method of the present invention.

FIG. 23 depicts a driving sequence utilizing the seventh driving methodof the present invention.

FIG. 24 illustrates an eighth driving method of the present invention.

FIG. 25 illustrates a driving sequence utilizing the eighth drivingmethod of the present invention.

SUMMARY OF THE INVENTION

A first aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side; and        repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i) and (ii) are repeated at least 4 times. In oneembodiment, the method further comprises a shaking waveform before step(i). In one embodiment, the method further comprises driving the pixelto the full color state of the first type of pigment particles after theshaking waveform but prior to step (i). In one embodiment, the firstperiod of time is 40 to 140 msec, the second period of time is greaterthan or equal to 460 msec and steps (i) and (ii) are repeated at leastseven times.

A second aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid sandwiched between a common electrode and a layer ofpixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and        repeating steps (i), (ii) and (iii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i), (ii) and (iii) are repeated at least 4 times. Inone embodiment, the method further comprises a shaking waveform beforestep (i). In one embodiment, the method further comprises a driving stepto the full color state of the first type of pigment particles after theshaking waveform but prior to step (i).

A third aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        and the method has a voltage insensitive range of at least 0.7V.

A fourth aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying no driving voltage to the pixel for a second        period of time;    -   (iii) applying a second driving voltage to the pixel for a third        period of time, wherein the second driving voltage is same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iv) applying no driving voltage to the pixel for a fourth        period of time; and        repeating steps (i)-(iv).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i)-(iv) are repeated at least 3 times. In oneembodiment, the method further comprises a shaking waveform before step(i). In one embodiment, the method further comprises driving the pixelto the full color state of the first type of pigment particles after theshaking waveform but prior to step (i).

A fifth aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the second type of pigment particles to drive the        pixel towards the color state of the second type of pigment        particles at the viewing side; and        repeating steps (i) and (ii).

In one embodiment, the method further comprises a wait time where nodriving voltage is applied. In one embodiment, the first type of pigmentparticles is negatively charged and the second type of pigment particlesis positively charged. In one embodiment, the second period of time isat least twice as long as the first period of time. In one embodiment,steps (i) and (ii) are repeated for least three times. In oneembodiment, the method further comprises a shaking waveform before step(i). In one embodiment, the method further comprises driving the pixelto the full color state of the second type of pigment particles afterthe shaking waveform but prior to step (i).

A sixth aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the second type        of pigment particles to drive the pixel towards the color state        of the second type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards the color state of the first type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and        repeating steps (i), (ii) and (iii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, steps (i), (ii) and (iii) are repeated at least threetimes. In one embodiment, the amplitude of the second driving voltage issame as that of the driving voltage required to drive the pixel from thecolor state of the first type of pigment particles to the color state ofthe second type of pigment particles, or vice versa. In one embodiment,the amplitude of the second driving voltage is higher than the amplitudeof the driving voltage required to drive the pixel from the color stateof the first type of pigment particles to the color stat of the secondtype of pigment particles, or vice versa. In one embodiment, the methodfurther comprises a shaking waveform. In one embodiment, the methodfurther comprises driving the pixel to the full color state of the firsttype of pigment particles after the shaking waveform but prior to step(i).

A seventh aspect of the present invention is directed to a drivingmethod for an electrophoretic display comprising a first surface on theviewing side, a second surface on the non-viewing side and anelectrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the second type of        pigment particles to drive the pixel towards the color state of        the second type of pigment particles wherein the first period of        time is not sufficient to drive the pixel to the full color        state of the second type of pigment particles at the viewing        side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards a mixed state of the first and second types of        pigment particles at the viewing side; and        repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i) and (ii) are repeated at least 4 times. In oneembodiment, the method further comprises a shaking waveform before step(i). In one embodiment, the method further comprises driving the pixelto the full color state of the first type of pigment particles after theshaking waveform but prior to step (i).

The fourth driving method of the present invention may be applied to apixel at a color state of the first type of pigment particles or may beapplied to a pixel at a color state not the color state of the firsttype of pigment particles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to driving methods for color displaydevices.

The device utilizes an electrophoretic fluid is shown in FIG. 1. Thefluid comprises three types of pigment particles dispersed in adielectric solvent or solvent mixture. For ease of illustration, thethree types of pigment particles may be referred to as white particles(11), black particles (12) and colored particles (13). The coloredparticles are non-white and non-black.

However, it is understood that the scope of the invention broadlyencompasses pigment particles of any colors as long as the three typesof pigment particles have visually distinguishable colors. Therefore,the three types of pigment particles may also be referred to as a firsttype of pigment particles, a second type of pigment particles and athird type of pigment particles.

For the white particles (11), they may be formed from an inorganicpigment, such as TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or thelike.

For the black particles (12), they may be formed from CI pigment black26 or 28 or the like (e.g., manganese ferrite black spinel or copperchromite black spinel) or carbon black.

The third type of particles may be of a color such as red, green, blue,magenta, cyan or yellow. The pigments for this type of particles mayinclude, but are not limited to, CI pigment PR 254, PR122, PR149, PG36,PG58, PG7, PB28, PB15:3, PY138, PY150, PY155 or PY20. Those are commonlyused organic pigments described in color index handbook “New PigmentApplication Technology” (CMC Publishing Co, Ltd, 1986) and “Printing InkTechnology” (CMC Publishing Co, Ltd, 1984). Specific examples includeClariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast redD3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm YellowH4G-EDS, Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia Red L4100 HD, and Irgazin Red L 3660 HD; Sun Chemical phthalocyanine blue,phthalocyanine green, diarylide yellow or diarylide AAOT yellow.

In addition to the colors, the first, second and third types ofparticles may have other distinct optical characteristics, such asoptical transmission, reflectance, luminescence or, in the case ofdisplays intended for machine reading, pseudo-color in the sense of achange in reflectance of electromagnetic wavelengths outside the visiblerange.

The solvent in which the three types of pigment particles are dispersedmay be clear and colorless. It preferably has a low viscosity and adielectric constant in the range of about 2 to about 30, preferablyabout 2 to about 15 for high particle mobility. Examples of suitabledielectric solvent include hydrocarbons such as isopar,decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil, silicon fluids, aromatic hydrocarbons such as toluene,xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene,halogenated solvents such as perfluorodecalin, perfluorotoluene,perfluoroxylene, dichlorobenzotrifluoride,3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene,dichlorononane or pentachlorobenzene, and perfluorinated solvents suchas FC-43, FC-70 or FC-5060 from 3M Company, St. Paul Minn., lowmolecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

A display layer utilizing the display fluid of the present invention hastwo surfaces, a first surface (16) on the viewing side and a secondsurface (17) on the opposite side of the first surface (16). The secondsurface therefore is on the non-viewing side. The term “viewing side”refers to the side at which images are viewed.

The display fluid is sandwiched between the two surfaces. On the side ofthe first surface (16), there is a common electrode (14) which is atransparent electrode layer (e.g., ITO), spreading over the entire topof the display layer. On the side of the second surface (17), there isan electrode layer (15) which comprises a plurality of pixel electrodes(15 a).

The display fluid is filled in display cells. The display cells may bealigned with or not aligned with the pixel electrodes. The term “displaycell” refers a micro-container which is filled with an electrophoreticfluid. Examples of “display cells” may include the cup-like microcellsas described in U.S. Pat. No. 6,930,818 and microcapsules as describedin U.S. Pat. No. 5,930,026. The micro-containers may be of any shapes orsizes, all of which are within the scope of the present application.

An area corresponding to a pixel electrode may be referred to as a pixel(or a sub-pixel). The driving of an area corresponding to a pixelelectrode is effected by applying a voltage potential difference (orknown as a driving voltage or an electric field) between the commonelectrode and the pixel electrode.

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

The space between two vertical dotted lines denotes a pixel (or asub-pixel). For brevity, when “pixel” is referred to in a drivingmethod, the term also encompasses “sub-pixel”s.

Two of the three types of pigment particles carry opposite chargepolarities and the third type of pigment particles is slightly charged.The term “slightly charged” or “lower charge intensity” is intended torefer to the charge level of the particles being less than about 50%,preferably about 5% to about 30%, the charge intensity of the strongercharged particles. In one embodiment, the charge intensity may bemeasured in terms of zeta potential. In one embodiment, the zetapotential is determined by Colloidal Dynamics AcoustoSizer IIM with aCSPU-100 signal processing unit, ESA EN# Attn flow through cell (K:127).The instrument constants, such as density of the solvent used in thesample, dielectric constant of the solvent, speed of sound in thesolvent, viscosity of the solvent, all of which at the testingtemperature (25° C.) are entered before testing. Pigment samples aredispersed in the solvent (which is usually a hydrocarbon fluid havingless than 12 carbon atoms), and diluted to between 5-10% by weight. Thesample also contains a charge control agent (Solsperse 17000®, availablefrom Lubrizol Corporation, a Berkshire Hathaway company; “Solsperse” isa Registered Trade Mark), with a weight ratio of 1:10 of the chargecontrol agent to the particles. The mass of the diluted sample isdetermined and the sample is then loaded into the flow through cell fordetermination of the zeta potential.

For example, if the black particles are positively charged and the whiteparticles are negatively charged, and then the colored pigment particlesmay be slightly charged. In other words, in this example, the chargescarried by the black and the white particles are much more intense thanthe charge carried by the colored particles.

In addition, the colored particles which carries a slight charge has acharge polarity which is the same as the charge polarity carried byeither one of the other two types of the stronger charged particles.

It is noted that among the three types of pigment particles, the onetype of particles which is slightly charged preferably has a largersize.

In addition, in the context of the present application, a high drivingvoltage (V_(H1) or V_(H2)) is defined as a driving voltage which issufficient to drive a pixel from one extreme color state to anotherextreme color state. If the first and the second types of pigmentparticles are the higher charged particles, a high driving voltage then(V_(H1) or V_(H2)) refers a driving voltage which is sufficient to drivea pixel from the color state of the first type of pigment particles tothe color state of the second type of pigment particles, or vice versa.For example, a high driving voltage, V_(H1), refers to a driving voltagewhich is sufficient to drive a pixel from the color state of the firsttype of pigment particles to the color state of the second type ofpigment particles, and V_(H2) refers to a driving voltage which issufficient to drive a pixel from the color state of the second type ofpigment particles to the color state of the first type of pigmentparticles. In this scenario as described, a low driving voltage (V_(L))is defined as a driving voltage which may be sufficient to drive a pixelto the color state of the third type of pigment particles (which areless charged and may be larger in size) from the color state of thefirst type of pigment particles. For example, a low driving voltage maybe sufficient to drive to the color state of the colored particles whilethe black and white particles are not seen at the viewing side.

In general, the V_(L) is less than 50%, or preferably less than 40%, ofthe amplitude of V_(H) (e.g., V_(H1) or V_(H2)).

The following is an example illustrating a driving scheme of howdifferent color states may be displayed by an electrophoretic fluid asdescribed above.

Example

This example is demonstrated in FIG. 2. The white pigment particles (21)are negatively charged while the black pigment particles (22) arepositively charged, and both types of the pigment particles are smallerthan the colored particles (23).

The colored particles (23) carry the same charge polarity as the blackparticles, but are slightly charged. As a result, the black particlesmove faster than the colored particles (23) under certain drivingvoltages.

In FIG. 2 a, the applied driving voltage is +15V (i.e., V_(H1)). In thiscase, the white particles (21) move to be near or at the pixel electrode(25) and the black particles (22) and the colored particles (23) move tobe near or at the common electrode (24). As a result, the black color isseen at the viewing side. The colored particles (23) move towards thecommon electrode (24) at the viewing side; however because their lowercharge intensity and larger size, they move slower than the blackparticles.

In FIG. 2 b, when a driving voltage of −15V (i.e., V_(H2)) is applied,the white particles (21) move to be near or at the common electrode (24)at the viewing side and the black particles and the colored particlesmove to be near or at the pixel electrode (25). As a result, the whitecolor is seen at the viewing side.

It is noted that V_(H1) and V_(H2) have opposite polarities, and havethe same amplitude or different amplitudes. In the example as shown inFIG. 2, V_(H1) is positive (the same polarity as the black particles)and V_(H2) is negative (the same polarity as the white particles)

In FIG. 2 c, when a low voltage which is sufficient to drive the coloredparticles to the viewing side and has the same polarity as the coloredparticles is applied, the white particles are pushed downwards and thecolored particles move up towards the common electrode (24) to reach theviewing side. The black particles cannot move to the viewing sidebecause of the low driving voltage which is not sufficient to separatethe two stronger and oppositely charged particles, i.e., the blackparticles and the white particles, from each other when the two types ofpigment particles meet.

The driving from the white color state in FIG. 2 b to the colored statein FIG. 2 c may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles (i.e., white), a second type of pigment particles(i.e., black) and a third type of pigment particles (i.e., colored), allof which are dispersed in a solvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises driving a pixel in the electrophoretic        display from the color state of the first type of pigment        particles towards the color state of the third type of pigment        particles by applying a low driving voltage which is sufficient        to drive the third type of pigment particles to the viewing side        while leaving the first and second types of pigment particles on        the non-viewing side and the polarity of the low driving voltage        applied is the same as the polarity of the third type of pigment        particles.

In order to drive a pixel to the color state of the third type ofpigment particles, i.e., red (see FIG. 2 c), the method starts from thecolor state of the first type of pigment particles, i.e., white (seeFIG. 2 b).

When the color of the third type of particles is seen at the viewingside, the other two types of the particles may be mixed at thenon-viewing side (side opposite of the viewing side), resulting in anintermediate color state between the colors of the first and secondtypes of particles. If the first and second types of particles are blackand white and the third type of particles is red, then in FIG. 2 c, whenthe red color is seen at the viewing side, a grey color is at thenon-viewing side.

The driving method ideally would ensure both color brightness (i.e.,preventing the black particles from being seen) and color purity (i.e.,preventing the white particles from being seen) in the scenario of FIG.2 c. However, in practice, this desired result is difficult to achievefor various reasons, including particle size distribution and particlecharge distribution.

One solution to this is the use of a shaking waveform prior to drivingfrom the color state of the first type of pigment particles (i.e.,white) to the color state of the third type of pigment particles (i.e.,red). The shaking waveform consists of repeating a pair of oppositedriving pulses for many cycles. For example, the shaking waveform mayconsist of a +15V pulse for 20 msec and a −15V pulse for 20 msec andsuch a pair of pulses is repeated for 50 times. The total time of such ashaking waveform would be 2000 msec. The notation, “msec”, stands formillisecond.

The shaking waveform may be applied to a pixel regardless of the opticalstate (black, white or red) prior to a driving voltage being applied.After the shaking waveform is applied, the optical state would not be apure white, pure black or pure red. Instead, the color state would befrom a mixture of the three types of pigment particles.

For the method as described above, a shaking waveform is applied priorto the pixel being driven to the color state of the first type ofpigment particles (i.e., white). With this added shaking waveform, eventhough the white state is measurably the same as that without theshaking waveform, the color state of the third type of pigment particles(i.e., red) would be significantly better than that without the shakingwaveform, on both color brightness and color purity. This is anindication of better separation of the white particles from the redparticles as well as better separation of the black particles from thered particles.

Each of the driving pulses in the shaking waveform is applied for notexceeding half of the driving time required for driving from the fullblack state to the full white state, or vice versa. For example, if ittakes 300 msec to drive a pixel from a full black state to a full whitestate, or vice versa, the shaking waveform may consist of positive andnegative pulses, each applied for not more than 150 msec. In practice,it is preferred that the pulses are shorter.

It is noted that in all of the drawings throughout this application, theshaking waveform is abbreviated (i.e., the number of pulses is fewerthan the actual number).

The driving method is shown in FIG. 3, in which a high negative drivingvoltage (V_(H2), e.g., −15V) is applied for a period of t2, to drive apixel towards a white state after a shaking waveform. From the whitestate, the pixel may be driven towards the colored state (i.e., red) byapplying a low positive voltage (V_(L), e.g., +5V) for a period of t3(that is, driving the pixel from FIG. 2 b to FIG. 2 c).

The driving period “t2” is a time period sufficient to drive a pixel tothe white state when V_(H2) is applied and the driving period “t3” is atime period sufficient to drive the pixel to the red state from thewhite state when V_(L) is applied. A driving voltage is preferablyapplied for a period of t1 before the shaking waveform to ensure DCbalance. The term “DC balance”, throughout this application, is intendedto mean that the driving voltages applied to a pixel is substantiallyzero when integrated over a period of time (e.g., the period of anentire waveform).

The First Driving Method:

The first driving method of the present invention is illustrated in FIG.4. It relates to a driving waveform which is used to replace the drivingperiod of t3 in FIG. 3.

In an initial step, a high negative driving voltage (V_(H2), e.g., −15V)is applied, which is followed by a positive driving voltage (+V′) todrive a pixel towards the red state. The amplitude of the +V′ is lessthan 50% of the amplitude of V_(H) (e.g., V_(H1) or V_(H2)).

In this driving waveform, a high negative driving voltage (V_(H2)) isapplied for a period of t4 to push the white particles towards theviewing side, which is then followed by applying a positive drivingvoltage of +V′ for a period of t5, which pulls the white particles downand pushes the red particles towards the viewing side.

In one embodiment, t4 may be in the range of 20-400 msec and t5 may be≧200 msec.

The waveform of FIG. 4 is repeated for at least 4 cycles (N≧4),preferably at least 8 cycles. The red color becomes more intense aftereach driving cycle.

The driving method of FIG. 4 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side; and        repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage.

As stated, the driving waveform as shown in FIG. 4 may be used toreplace the driving period of t3 in FIG. 3 (see FIG. 5). In other words,the driving sequence may be: shaking waveform, followed by drivingtowards the white state for a period of t2 and then applying thewaveform of FIG. 4.

In another embodiment, the step of driving to the white state for aperiod of t2 may be eliminated and in this case, a shaking waveform isapplied before applying the waveform of FIG. 4 (see FIG. 6).

In one embodiment, the driving sequence of FIG. 5 or FIG. 6 is DCbalanced.

The Second Driving Method:

The second driving method the present invention is illustrated in FIG.7. It relates to an alternative to the driving waveform of FIG. 4, whichmay also be used to replace the driving period of t3 in FIG. 3.

In this alternative waveform, there is a wait time “t6” added. Duringthe wait time, no driving voltage is applied. The entire waveform ofFIG. 7 is also repeated for multiple cycles (for example, N≧4).

The waveform of FIG. 7 is designed to release the charge imbalancestored in the dielectric layers in an electrophoretic display device,especially when the resistance of the dielectric layers is high, forexample, at a low temperature.

In the context of the present application, the term “low temperature”refers to a temperature below about 10° C.

The wait time presumably can dissipate the unwanted charge stored in thedielectric layers and cause the short pulse (“t4”) for driving a pixeltowards the white state and the longer pulse (“t5”) for driving thepixel towards the red state to be more efficient. As a result, thisalternative driving method will bring a better separation of the lowcharged pigment particles from the higher charged ones. The wait time(“t6”) can be in a range of 5-5,000 msec, depending on the resistance ofthe dielectric layers.

This driving method of FIG. 7 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid which fluid is sandwichedbetween a common electrode and a layer of pixel electrodes and comprisesa first type of pigment particles, a second type of pigment particlesand a third type of pigment particles, all of which are dispersed in asolvent or solvent mixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and        repeating steps (i), (ii) and (iii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage.

As stated, the driving waveform as shown in FIG. 7 may also be used toreplace the driving period of t3 in FIG. 3 (see FIG. 8). In other words,the driving sequence may be: shaking waveform, followed by drivingtowards the white state for a period of t2 and then applying thewaveform of FIG. 7.

In another embodiment, the step of driving to the white state for aperiod of t2 may be eliminated and in this case, a shaking waveform isapplied before applying the waveform of FIG. 7 (see FIG. 9).

In another embodiment, the driving sequence of FIG. 8 or FIG. 9 is DCbalanced.

It should be noted that the lengths of any of the driving periodsreferred to in this application may be temperature dependent.

The Third Driving Method:

FIG. 10 a demonstrates the relationship between applied driving voltage(V′) and the optical performance, based on the waveform of FIG. 3. Asshown, the positive driving voltage V′ applied may impact on the redstate performance of a color display device described above. The redstate performance of the display device is expressed as a* value,utilizing the L*a*b* color system.

The maximum a* in FIG. 10 a appears at the applied driving voltage V′,in FIG. 3, being about 3.8V. However, if a change of ±0.5V is made tothe applied driving voltage, the resulting a* value would be about 37which is roughly 90% of the maximum a*, thus still acceptable. Thistolerance can be beneficial to accommodate changing of the drivingvoltages caused by, for example, variation in the electronic componentsof a display device, the drop of battery voltage over time, batchvariation of the TFT backplanes, batch variation of the display devicesor temperature and humidity fluctuations.

Based on the concept of FIG. 10 a, a study was performed to find a rangeof driving voltages V′ that can drive to the red state with an over 90%of the maximum a* value. In other words, when any of the drivingvoltages in the range is applied, the optical performance is notsignificantly affected. Therefore, the range may be referred to as“voltage-insensitive” range”. The wider the “voltage insensitive” range,the more tolerant the driving method is to batch variations andenvironmental changes.

In FIG. 4, there are three parameters to be considered for this study,t4, t5 and N. The effects of the three parameters on thevoltage-insensitive range are interactive and non-linear.

Following the model of FIG. 10 a, one can find the optimum value setsfor the three parameters to achieve the widest voltage-insensitive rangefor the waveform of FIG. 4. The results are summarized in FIG. 10 b.

In one example, when t4 is between 40˜140 msec, t5 is greater than orequal to 460 msec and N is greater than or equal to 7, thevoltage-insensitive range (i.e., 3.7V to 6.5V) based on FIG. 10 b istwice the width of the voltage-insensitive range (i.e., 3.3V-4.7V) basedon FIG. 10 a.

The optimized parameters discussed above are also applicable to any ofthe driving methods of the present invention.

The third driving method therefore may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrode and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        and the method has a voltage insensitive range of at least 0.7V.

In such a method, when a driving voltage within such a range is applied,the optical quality of a color state achieved is at least 90% of themaximum acceptable “a*” value.

It is also noted that the data shown in FIGS. 10 a and 10 b arecollected under ambient temperature.

The Fourth Driving Method:

The fourth driving method of the present invention is illustrated inFIG. 11. It relates to a driving waveform which may also be used toreplace the driving period of t3 in FIG. 3.

In an initial step, a high negative driving voltage (V_(H2), e.g., −15V)is applied to a pixel for a period of t7, which is followed by a waittime of t8. After the wait time, a positive driving voltage (V′, e.g.,less than 50% of V_(H1) or V_(H2)) is applied to the pixel for a periodof t9, which is followed by a second wait time of t10. The waveform ofFIG. 11 is repeated N times. The term, “wait time”, as described above,refers to a period of time in which no driving voltage is applied.

This driving method not only is particularly effective at a lowtemperature, it can also provide a display device better tolerance ofstructural variations caused during manufacture of the display device.Therefore its usefulness is not limited to low temperature driving.

In the waveform of FIG. 11, the first wait time t8 is very short whilethe second wait time t10 is longer. The period of t7 is also shorterthan the period of t9. For example, t7 may be in the range of 20-200msec; t8 may be less than 100 msec; t9 may be in the range of 100-200msec; and t10 may be less than 1000 msec.

FIG. 12 is a combination of FIG. 3 and FIG. 11. In FIG. 3, a white stateis displayed during the period of t2. As a general rule, the better thewhite state in this period, the better the red state that will bedisplayed at the end.

In the shaking waveform, the positive/negative pulse pair is preferablyrepeated 50-1500 times and each pulse is preferably applied for 10 msec.

In one embodiment, the step of driving to the white state for a periodof t2 may be eliminated and in this case, a shaking waveform is appliedbefore applying the waveform of FIG. 11 (see FIG. 13).

The fourth driving method of FIG. 11 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying no driving voltage to the pixel for a second        period of time;    -   (iii) applying a second driving voltage to the pixel for a third        period of time, wherein the second driving voltage has same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iv) applying no driving voltage to the pixel for a fourth        period of time; and        repeating steps (i)-(iv).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, steps (i)-(iv) are repeated at least 3 times.

In one embodiment, the second driving voltage is less than 50% of thedriving voltage sufficient to drive a pixel from the color state of thefirst type of pigment particles to the color state of the second type ofpigment particles, or vice versa.

In another embodiment, the driving sequence of FIG. 12 or FIG. 13 is DCbalanced.

The Fifth Driving Method:

As shown in FIG. 2( a), because the black particles and the redparticles carry the same charge polarity, they tend to move in the samedirection. Even though the black particles move faster than the redparticles under certain driving voltages because of their higher chargeand possibly also smaller size, some of the red particles may still bedriven to the viewing side with the black particles, to cause thequality of the black state to degrade.

FIG. 14 depicts a typical waveform for driving a pixel towards the blackstate. A shaking waveform (explained above) is included to ensure colorbrightness and purity. As shown, a high positive driving voltage(V_(H1), e.g., +15V) is applied for a period of t12 to drive a pixeltowards a black state after the shaking waveform. A driving voltage isapplied for a period of t11 before the shaking waveform to ensure DCbalance.

The fifth driving method of the present invention is illustrated in FIG.15. It relates to a driving waveform to be added at the end of thewaveform of FIG. 14, for driving a pixel towards the black state. Thecombined waveform can further provide better separation of the blackparticles from the red particles, rendering the black state moresaturated, with less red tinting.

In FIG. 15, a short pulse “t13” of V_(H2) (negative) is applied,followed by a longer pulse “t14” of V_(H1) (positive) and a wait time(0V) of t15. Such a sequence is applied for at least once, preferably atleast 3 times (i.e., N is ≧3) and more preferably at least five to seventimes.

The pulse “t14” is usually at least twice the length of the pulse “t13”.

The short pulse “t13” of V_(H2) will push the black and red particlestowards the pixel electrode and the longer pulse “t14” of V_(H1) willpush them to the common electrode side (i.e., the viewing side). Sincethe speed of the two types of pigment particles are not the same underthe same driving voltages, this asymmetrical driving sequence willbenefit the black particles more than the red particles. As a result,the black particles can be better separated from the red particles.

The wait time “t15” is optional, depending on the dielectric layers inthe display device. It is common that at a lower temperature, theresistance of the dielectric layers is more pronounced and, in thiscase, a wait time may be needed to release the charge trapped in thedielectric layers.

The fifth driving method of FIG. 15 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the second type of pigment particles to drive the        pixel towards the color state of the second type of pigment        particles at the viewing side;    -   (iii) optionally applying no driving voltage to the pixel for a        third period of time;        and        repeating steps (i), (ii) and (iii) if present.

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

FIG. 16 shows the sequence combining the waveform of FIG. 14 and thewaveform of FIG. 15. However it is also noted that, depending on theparticle speed and the cycle number (N) of the sequence, “t12” may beshortened. In other words, at the end of “t12”, the pixel does not haveto be at the full black state. Instead, the waveform of FIG. 15 couldstart at any state from black to white, including grey, provided thatthe number (N) in the sequence is sufficient to drive the pixel to theblack state at the end.

The method as described in FIGS. 14-16 may also be utilized to drive apixel to the black state at a low temperature. In this case, the periodt14 must be longer than t13 and the wait time t15 has to be at least 50msec.

In one embodiment, the driving sequence of FIG. 16 is DC balanced.

The Sixth Driving Method:

FIG. 17 depicts a typical waveform for driving a pixel to a white state.A shaking waveform (explained above) is included to ensure colorbrightness and purity. A driving voltage of V_(H2) is applied for aperiod of t17 after the shaking waveform. A driving voltage is appliedfor a period of t16 before the shaking waveform to ensure DC balance.

The sixth driving method of the present invention is illustrated inFIGS. 18( a) and 18(b). It relates to waveforms to replace t17 in thewaveform of FIG. 17.

This driving method is particularly suitable for low temperaturedriving, although it is not limited to low temperature driving.

In FIG. 18( a), a short pulse “t18” of V_(H1) (positive) is applied,followed by a longer pulse “t19” of V_(H2) (negative) and a wait time(0V) of t20. As shown in FIG. 18( b), the amplitude of the negativedriving voltage (V″) applied during t19 may be higher than that ofV_(H2) (e.g., −30V instead of −15V).

Such a sequence is applied for at least once, preferably at least 3times (i.e., N is ≧3 in FIGS. 18( a) and 18(b), and more preferably atleast five to seven times.

It is noted that the t19 must be longer than t18. For example, t18 maybe in the range of 20-200 msec and t19 may be less than 1000 msec. Thewait time t20 needs to be at least 50 msec.

The sixth driving method as shown in FIGS. 18( a) and 18(b) may besummarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the second type        of pigment particles to drive the pixel towards the color state        of the second type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards the color state of the first type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and        repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment as shown in FIG. 18( a), the second voltage is thedriving voltage required to drive a pixel from the color state of thefirst type of pigment particles towards the color state of the secondtype of pigment particles, or vice versa.

In another embodiment as shown in FIG. 18( b), the second voltage has aamplitude higher than that of the driving voltage required to drive apixel from the color state of the first type of pigment particlestowards the color state of the second type of pigment particles, or viceversa.

FIGS. 19 a and 19 b show sequences where t17 in FIG. 17 is replaced witha waveform of FIGS. 18( a) and 18(b), respectively.

In the shaking waveform, the positive/negative pulse pair is preferablyrepeated 50-1500 times and each pulse is preferably applied for 10 msec.

In one embodiment, the driving sequence of FIG. 19 a or FIG. 19 b is DCbalanced.

The Seventh Driving Method:

The seventh driving method of the present invention drives a pixeltowards an intermediate color state (e.g., grey).

FIG. 20 illustrates the driving scheme. As shown, a pixel in the blackstate (see FIG. 20 a) is driven towards a grey state when a low negativedriving voltage (V_(L), e.g., −5V) is applied. In the process, the lowdriving voltage pushes the red particles towards the side of the pixelelectrode and a mixture of black and white particles is seen at theviewing side.

This driving method is shown in FIG. 21. A high positive driving voltage(V_(H1), e.g., +15V) is applied for a time period of t22 to drive apixel towards a black state, after a shaking waveform. From the blackstate, the pixel may be driven towards the grey state by applying a lownegative driving voltage (V_(L), e.g., −5V) for a period of t23, thatis, driven from FIG. 20( a) to FIG. 20( b).

The driving period t22 is a time period sufficient to drive a pixel tothe black state when V_(H1) is applied, and t23 is a time periodsufficient to drive the pixel to the grey state from the black statewhen V_(L) is applied. Prior to the shaking waveform, a pulse of V_(H1)is preferably applied for a period of t21 to ensure DC balance.

FIG. 22 relates to a driving waveform which may be used to replace thedriving period of t23 in FIG. 21. In an initial step, a high positivedriving voltage (V_(H1), e.g., +15V) is applied for a short period oft24 to push the black particles towards the viewing side, but t24 is notsufficient to drive the pixel to the full black state, which is followedby applying a low negative driving voltage (V_(L), e.g., −5V) for aperiod of t25 to drive the pixel towards a grey state. The amplitude ofV_(L) is less than 50% of V_(H) (e.g., V_(H1) or V_(H2)).

The waveform of FIG. 22 is repeated for at least 4 cycles (N≦4),preferably at least 8 cycles.

The time period, t24 is less than about 100 msec and t25 is usuallygreater than 100 msec, both at ambient temperature.

The seventh driving method as shown in FIG. 22 may be summarized asfollows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the second type of        pigment particles to drive the pixel towards the color state of        the second type of pigment particles wherein the first period of        time is not sufficient to drive the pixel to the full color        state of the second type of pigment particles at the viewing        side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards a mixed state of the first and second types of        pigment particles at the viewing side; and        repeating steps (i) and (ii).

As stated above, the second driving voltage is about 50% of the firstdriving voltage, in this method.

FIG. 23 shows the combination of the waveform of FIG. 21 and thewaveform of FIG. 22, in which the driving period of t23 is replaced withFIG. 22. In other words, the driving method may consist of four phases.The first phase is the DC balance phase (t21); the second phase is ashaking step; and the third phase is driving a pixel to the black state(t22). In the third phase, the waveform can be any waveform as long asit drives a pixel to a good black state. The fourth phase consists of ahigh positive driving voltage for a short period of time, followed by alow negative driving voltage for a longer period of time. The fourthphase, as stated, is repeated several times.

It is noted that in FIG. 23, t22 may be optional.

It is possible to modulate the grey state to be brighter or darker bychanging the low negative voltage (V_(L)). In other words, the waveformsequence and shape may remain the same; but the amplitude of V_(L)varies (e.g. −4V, −5V, −6V or −7V) to cause different grey levels to bedisplayed. This feature could potentially reduce the required space forthe look-up tables in the driving circuit, consequently lowering thecost. The driving method as illustrated can produce a high quality of anintermediate state (of the first type of pigment particles and thesecond type of pigment particles) with very little color interferencefrom the third type of pigment particles.

In one embodiment, the driving sequence of FIG. 23 is DC balanced.

The Eighth Driving Method:

The eighth driving method of the present invention is illustrated inFIG. 24. It is intended to be applied to a pixel which is not at a whitestate (i.e., the color state of the first type of pigment particles).

In an initial step, a high negative driving voltage (V_(H2), e.g., −15V)is applied for a period of t26, which is followed by a wait time of t27.After the wait time, a positive driving voltage (V′, e.g., less than 50%of V_(H1) or V_(H2)) is applied for a period of t28, which is followedby a second wait time of t29. The waveform of FIG. 24 is repeated Ntimes. The term, “wait time”, as described above, refers to a period oftime in which no driving voltage is applied.

This driving method is particularly effective at a low temperature, andit may also shorten the overall driving time to the red state.

It is noted that the time period t26 is rather short, usually in therange of about 50% of the time required to drive from a full black stateto a full white state and therefore it is not sufficient to drive apixel to a full white color state. The time period t27 may be less than100 msec; the time period t28 may range of 100-200 msec; and the timeperiod t29 may be less than 1000 msec.

It is also noted that the waveform of FIG. 24 is similar to that of FIG.11, except that the waveform of FIG. 11 is disclosed to be applied to apixel which is in a white state (i.e., the color of the first type ofpigment particles) whereas the waveform of FIG. 24 is intended to beapplied to a pixel which is not in a white state.

FIG. 25 is an example wherein the waveform of FIG. 24 is applied to apixel which is at a black state (i.e., the color state of the secondtype of pigment particles).

In the shaking waveform, the positive/negative pulse pair is preferablyrepeated 50-1500 times and each pulse is preferably applied for 10 msec.

The eighth driving method of FIG. 24, like that of FIG. 11, may besummarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   (a) the three types of pigment particles have optical        characteristics differing from one another;    -   (b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   (c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying no driving voltage to the pixel for a second        period of time;    -   (iii) applying a second driving voltage to the pixel for a third        period of time, wherein the second driving voltage has same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iv) applying no driving voltage to the pixel for a fourth        period of time; and        repeating steps (i)-(iv).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, steps (i)-(iv) are repeated at least 3 times.

In one embodiment, the second driving voltage is less than 50% of thedriving voltage sufficient to drive a pixel from the color state of thefirst type of pigment particles to the color state of the second type ofpigment particles, or vice versa.

In one embodiment, the driving sequence of FIG. 25 is DC balanced.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

What is claimed is:
 1. A driving method for an electrophoretic displaycomprising a first surface on the viewing side, a second surface on thenon-viewing side and an electrophoretic fluid which fluid is sandwichedbetween a common electrode and a layer of pixel electrodes and comprisesa first type of pigment particles, a second type of pigment particlesand a third type of pigment particles, all of which are dispersed in asolvent or solvent mixture, wherein (a) the three types of pigmentparticles have optical characteristics differing from one another; (b)the first type of pigment particles and the second type of pigmentparticles carry opposite charge polarities; and (c) the third type ofpigment particles has the same charge polarity as the second type ofpigment particles but at a lower intensity, the method comprises thefollowing steps: (i) applying a first driving voltage to a pixel in theelectrophoretic display for a first period of time, wherein the firstdriving voltage has the same polarity as the first type of pigmentparticles to drive the pixel towards the color state of the first typeof pigment particles at the viewing side; (ii) applying a second drivingvoltage to the pixel for a second period of time, wherein the seconddriving voltage has the same polarity as the third type of pigmentparticles to drive the pixel towards the color state of the third typeof pigment particles at the viewing side; and repeating steps (i) and(ii).
 2. The method of claim 1, wherein the first type of pigmentparticles is negatively charged and the second type of pigment particlesis positively charged.
 3. The method of claim 1, wherein the amplitudeof the second driving voltage is less than 50% of the amplitude of thefirst driving voltage.
 4. The method of claim 1, wherein steps (i) and(ii) are repeated at least 4 times.
 5. The method of claim 1, furthercomprising a shaking waveform before step (i).
 6. The method of claim 5,further comprising a driving step to the full color state of the firsttype of pigment particles after the shaking waveform but prior to step(i).
 7. A driving method for an electrophoretic display comprising afirst surface on the viewing side, a second surface on the non-viewingside and an electrophoretic fluid which fluid sandwiched between acommon electrode and a layer of pixel electrodes and comprises a firsttype of pigment particles, a second type of pigment particles and athird type of pigment particles, all of which are dispersed in a solventor solvent mixture, wherein (a) the three types of pigment particleshave optical characteristics differing from one another; (b) the firsttype of pigment particles and the second type of pigment particles carryopposite charge polarities; and (c) the third type of pigment particleshas the same charge polarity as the second type of pigment particles butat a lower intensity, the method comprises the following steps (i)applying a first driving voltage to a pixel in the electrophoreticdisplay for a first period of time, wherein the first driving voltagehas the same polarity as the first type of pigment particles to drivethe pixel towards the color state of the first type of pigment particlesat the viewing side; (ii) applying a second driving voltage to the pixelfor a second period of time, wherein the second driving voltage has thesame polarity as the third type of pigment particles to drive the pixeltowards the color state of the third type of pigment particles at theviewing side; (iii) applying no driving voltage to the pixel for a thirdperiod of time; and repeating steps (i), (ii) and (iii).
 8. The methodof claim 7, wherein the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.9. The method of claim 7, wherein the amplitude of the second drivingvoltage is less than 50% of the amplitude of the first driving voltage.10. The method of claim 7, wherein steps (i), (ii) and (iii) arerepeated at least 4 times.
 11. The method of claim 7, further comprisinga shaking waveform before step (i).
 12. The method of claim 11, furthercomprising a driving step to the full color state of the first type ofpigment particles after the shaking waveform but prior to step (i). 13.The method of claim 1, wherein the first period of time is 40 to 140msec, the second period of time is greater than or equal to 460 msec andsteps (i) and (ii) are repeated at least seven times.
 14. A drivingmethod for an electrophoretic display comprising a first surface on theviewing side, a second surface on the non-viewing side and anelectrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein (a) the three types of pigment particles have opticalcharacteristics differing from one another; (b) the first type ofpigment particles and the second type of pigment particles carryopposite charge polarities; and (c) the third type of pigment particleshas the same charge polarity as the second type of pigment particles butat a lower intensity, the method comprises the following steps: (i)applying a first driving voltage to a pixel in the electrophoreticdisplay for a first period of time, wherein the first driving voltagehas the same polarity as the first type of pigment particles to drivethe pixel towards the color state of the first type of pigment particlesat the viewing side; (ii) applying no driving voltage to the pixel for asecond period of time; (iii) applying a second driving voltage to thepixel for a third period of time, wherein the second driving voltage issame polarity as the third type of pigment particles to drive the pixeltowards the color state of the third type of pigment particles at theviewing side; (iv) applying no driving voltage to the pixel for a fourthperiod of time; and repeating steps (i)-(iv).
 15. The method of claim14, wherein the first type of pigment particles is negatively chargedand the second type of pigment particles is positively charged.
 16. Themethod of claim 14, wherein the amplitude of the second driving voltageis less than 50% of the amplitude of the first driving voltage.
 17. Themethod of claim 14, wherein steps (i)-(iv) are repeated at least 3times.
 18. The method of claim 14, further comprising a shaking waveformbefore step (i).
 19. The method of claim 18, further comprising adriving step to the full color state of the first type of pigmentparticles after the shaking waveform but prior to step (i).